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    <blockquote> <p>I could write anal setters that will tolerate themselves being called in many orders but that will not solve the problem of a user forgetting to set the appropriate setter and therefore the bean being in an incomplete state.</p> </blockquote> <p>The Spring-specific solution to this is to declare the bean as implementing <code>InitializingBean</code> and then implement the necessary checking and back-filling of defaults in the bean's <code>afterPropertiesSet()</code> method. You can even wire this up so that programmers have the choice of using constructors with arguments or setters, and (maybe) have the argument-full constructors call <code>afterPropertiesSet()</code>.</p> <p>The only gotchas are:</p> <ul> <li><p>your codebase becomes Spring dependent by virtue of implementing <code>InitializingBean</code>, and</p></li> <li><p>if a developer configures the beans by hand using the setters they have to remember to call <code>afterPropertiesSet()</code> when they are done.</p></li> </ul> <p><strong>EDIT</strong></p> <p>The <code>afterPropertiesSet()</code> method also gives a neat solution to your example with calculating the id. The problem with the way you have currently written it is that it implicitly constrains you to building the tree from the top (root) down. For example, suppose you have a multi-level tree, and you set a leaf node's parent attribute <em>before</em> you set the parent node's parent attribute. This will trigger the <code>id</code> setting algorithm in the leaf node, which will wrongly think that the parent node is a root node.</p> <p>In Spring you can arrange that the setting of the <code>parent</code> attributes is done in the implementation of the relevant setChild / setChildren methods of the parent nodes. The setting of the <code>id</code> attributes would then be done in the <code>afterPropertiesSet()</code> method. Since this gets called <em>after</em> all explicit and auto-wire property settings have been applied, the parent/child relationships for the tree will have stabilized, and so the <code>id</code> calculations will be correct.</p>
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    1. POConstructor versus setter injection
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    1. COActually, I was considering asking this as part of the original question but glad to see that you've answered it anyway. I wouldn't be enthusiastic about adding Spring-Specific annotations to the code. My feeling is that the interface itself should be responsible for prohibiting non-intended use so that the protection is afforded to all usage vectors of the API (Spring/Java/PicoContainer/etc.).
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    2. COYou don't need to add annotations to use Spring. I use XML config files exclusively in my projects. Also, I would be surprised if you figure out something that works well across a range of DI systems.
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    3. COAlso, I'm not sure you can control the order in which the Spring wiring frameworks calls your bean's setters.
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